The present invention is broadly applicable to the deposition of a metallic plating on polymeric plastic substrates, and more particularly to an improved initiator solution and process for treating copper and copper alloy coatings, to render them receptive to the electroless deposition of nickel on copper and copper alloy coatings.
An electroless copper plating followed by an electroless nickel plating has been found to be a very useful composite metallic coating on polymeric substrates such as electrical component housings because of the excellent electromagnetic interference (EMI) shielding which is provided by such coating. The deposition of about 20 to about 40 microinches of electroless copper on such a polymeric substrate provides the requisite EMI shielding and the subsequent electroless nickel overplate provides for improved corrosion resistance and decorative appearance of the composite plate.
A continuing problem has been encountered in effecting a uniform electroless nickel deposit on copper or copper alloy substrates in that the electroless nickel process is somewhat unpredictable depending upon the specific type of electroless nickel process employed. Generally, electroless nickel processes can be categorized as being of the high temperature (175.degree. to 212.degree. F.) acid process and the low temperature (75.degree. to 160.degree. F.) alkaline process. Initiation of a nickel deposit will occur much faster from hot acid electroless nickel processes than in lower temperature alkaline electroless nickel processes in which the initiation time may exceed 30 minutes which is commercially unsatisfactory. The initiation time as herein used is defined as the period commencing when a copper substrate is immersed into the bath and ending when the nickel plating commences. In order to reduce the initiation time, three general methods are currently in use to effect fairly rapid initiation of an electroless nickel plate on copper and copper alloy surfaces which include the electrolytic deposition of an initial nickel strike, the direct reduction method in which the copper substrate is contacted with a reducing agent, and, finally, the cathodic cell method employing an aqueous solution containing noble metal ions such as gold or palladium to deposit catalytic reduction sites on the copper substrate by chemical displacement. Of the foregoing, the cathodic cell method is considered technically superior and is better adapted for high volume production installations.
In accordance with prior cathodic cell initiator techniques, a noble metal, particularly palladium is employed in the aqueous solution in which a previously electroless copper plated substrate is immersed for a controlled time period. It has been found, however, that after only several days of heavy production use, certain such palladium containing initiator solutions become ineffective necessitating disposal and replacement. Because of the relatively low concentration of palladium in the initiator solution, it is not presently economical to subject such waste initiator solutions to recovery of the valuable palladium constituents therein. In addition to the cost and waste disposal problems associated with the discarding of such spent initiator solutions, the progressive degradation of such solutions during use also causes erratic and unpredictable behavior of the substrates being treated in the electroless nickel plating solution detracting from the attainment of uniform nickel deposits thereon.
The present invention overcomes the problems and cost disadvantages associated with prior art noble metal initiator solutions by providing a composition which provides for a stabilization and prolonged effective operating life of such initiator solutions as well as a process for rejuvenating such initiator solutions which have been rendered ineffective or impaired due to the progressive contamination thereof particularly by organic metal ion complexing agents.